Crystal structure refinement of mendipite, Pb3O2Cl2

The crystal structure of mendipite from the Mendip Hills (UK) was refined in the space group Pnma, using single-crystal X-ray diffraction data, to R-F = 0.078. The refined unit cell parameters are a = 11.905(2), b = 5.817(1), c = 9.542(2) Angstrom, Z = 4. The structure corresponds to that of synthetic Pb3O2Cl2, and differs from the previously proposed model for natural mendipite. In mendipite three independent and irregular [Pb phi (6)] and [Pb phi (7)] polyhedra (phi = generic anion) occur. Cation-anion distances are spread over a wide range, due to the lone-pair effect of Pb2+, resulting in (4+2) and (4+3) coordinations for the lead atoms. In the structure of mendipite, columnar arrangement of lead-centered polyhedra is realized, and columns alternate with structural voids